Twin-screw extruders can be driven by a pair of meshing spur gears which are simply supported at opposite ends in journal bearings, advantageously of the roller type. With heavy-duty extruders, in which the spur gears must sustain considerable axial as well as flexural stresses, the spur gears are frequently provided with herringbone teeth and with supplemental intermediate journal bearings, the latter receiving tooth-free median zones separating the helically toothed gear sections of opposite pitch. Heretofore, these intermediate bearings had to be split to facilitate their mounting in a stationary support, such as a partition of a cylindrical gear housing; plain bearings with bronze linings were normally used for this purpose. The effective lubrication of such bearings, with their low journal speeds, generally requires a forced circulation of oil for which the energy must be supplied by the input shaft driving one of the meshing gears. Such an arrangement not only entails additional expenses but also subjects the driving and driven gears to unbalanced torques, thereby limiting the axial extent of the area of effective mesh.
An earlier proposal designed to enable the use of undivided anti-friction journal bearings at intermediate locations involves the axial subdivision of each gear into separate sections which are positively coupled with one another after assembly and may be supported on a common shaft. Such a system, too, is relatively complex and correspondingly expensive.